Microencapsulation techniques have been widely applied as one of means for enclosing (or sealing) various materials (or core materials) such as a dye, a perfume (aromatic or flavoring agent), a crystalline liquid, an enzyme, a catalyst, and an adhesive. The advantages of such techniques are in that the handleability of these core materials can be improved and that the functions of the core materials can be maintained or retained for a long period of time.
On the other hand, display techniques are utilized in a broad range from a displaying method for displaying an image or character information to a visualizing method using a mode such as a liquid crystal mode, a plasma emission mode, or an EL (electroluminescence) mode. In recent years, as various electronic apparatuses (or devices) are miniaturized due to rapid advance of semiconductor technology, there are increasing demands for the miniaturization, weight-saving, lower driving voltage, less electricity consumption to work, and thinner flat panel of display devices. As new display method for responding to these requirements, there are proposed electrophoretically image-displaying devices (or apparatuses) capable of writing images on the display surface by encapsulating microcapsules in a dispersed system (core material) in which electrophoretic particles (or electrophoretically-movable particles) are dispersed in a disperse medium, and interposing these microcapsules between electrode plates to migrate or move the electrophoretic particles in the microcapsules between these electrode plates by applying an electric field.
Japanese Patent Application Laid-Open No. 119264/1999 (JP-11-119264A) discloses a display device comprising a disperse system in which charged particles are dispersed into a disperse medium, a number of microcapsules encapsulating the disperse system, and a pair of opposed electrodes which are so disposed as to insert these microcapsules therebetween. In the display device, a given display operation is conducted by changing the distribution condition of the charged particles depending on an action of a controlled voltage to change the optical reflexivity. The particle size of the charged particles is about 1/1000 to ⅕ relative to that of the microcapsules, and the dispersivity in the particle size distribution of the charged particles (volume-average particle size/number-average particle size) is 1 to 2. Japanese Patent Application Laid-Open No. 202372/1999 (JP-11-202372A) discloses a display device comprising a disperse system comprising at least two kinds of charged particles encapsulated in the microcapsule, and a disperse medium containing a surfactant, wherein the charged particles contain at least one member of titanium oxide and carbon black.
Japanese Patent No. 2551783 discloses an electrophoretic display device using microcapsules encapsulating a disperse system, as microcapsules disposed between the electrodes, wherein the disperse system comprises a colored disperse medium, and at least one kind of an electrophoretic particle, dispersed in the medium, different in optical property from the medium. Further, Japanese Patent Application Laid-Open No. 503873/2001 (JP-2001-503873A) discloses an electrophoretically displaying device comprising an arrangement of discrete microscopic containers (or microcapsules); first and second electrodes disposed on and covering opposite sides of the arrangement, at least one of the electrodes being substantially visually transparent; a means for creating a potential difference between the two electrodes; and within each container, a suspension comprising a dielectric liquid and particles exhibiting surface charges in the dielectric liquid, wherein the dielectric liquid and the particles contracting visually, and the potential difference causing the particles to migrate toward one of the electrodes.
In such a microcapsule, the wall of the microcapsule should be dense or close to encapsulate a liquid as a core material. As a process for producing a microcapsule, physicochemical processes and chemical processes are known, and these processes have been suitably selected and utilized depending on applications of the microcapsule. As the physicochemical processes, a coacervation method using gelatin is well known, and is detailed in “Microcapsules (new edition, 1987)” attributed by Kondo Tamotsu et al. (published by Sankyoshuppan Co., Ltd.). Although this method is applied in a wide field, use of gelatin being a natural product causes variation of quality as a membrane material and deteriorates water resistance of the capsule, as a result the application is limited. Moreover, it is impossible to inhibit the formation of a core material-free coacervate particle/drop (as the core material, e.g., a coloring agent dispersed in oil), or the by-production of a capsule enclosing a plurality of core materials. Further, the coacervation method itself is a method for forming a capsule wall and fails to control the particle size of the resulting particles, and the particle size distribution of the particles depends on the dispersiveness of the core material. Therefore, according to the coacervation method, it is difficult to obtain a microcapsule encapsulating a disperse system in which a coloring agent is dispersed in an oil phase in high yield, while controlling the particle size and the particle size distribution.
As the chemical processes, in addition to an in-situ polymerization method (phase-separation method) which comprises allowing a reaction to proceed from a continuous aqueous phase for forming a wall made of an amino resin or others around a core material, there is known an interfacial polymerization in which a reaction component in an aqueous phase is reacted with another reaction component in an oil phase by polymerization or condensation at the phase interface for forming a polymeric wall of the microcapsule. In a production process of an encapsulated ink encapsulating a disperse system in which a coloring agent is dispersed in an oil phase, an in-situ polymerization method using an amino resin is particularly utilized. For example, Japanese Patent Publication No. 27452/1993 (JP-5-27452B) discloses a process for producing a microcapsule, which comprises polymerizing an initial condensate of a hydrophilic melamine-formaldehyde-series resin contained in a water or hydrophilic medium having an emulsifying agent, to coat a hydrophobic core material with the polymer product, wherein an acrylic copolymer is used as the emulsifying agent. Japanese Patent Publication No. 51339/1993 (JP-5-51339B) discloses a process for producing a microcapsule which comprises coating a hydrophobic material with a urea-formaldehyde resin coat in an acidic aqueous medium containing an anionic water-soluble polymer, wherein an acrylic copolymer is used as the anionic water-soluble material. Japanese Patent Publication No. 53538/1993 (JP-5-53538B) discloses a process for producing a microcapsule, which comprises dispersing or emulsifying a hydrophobic core material in an acidic aqueous solution containing an anionic water-soluble polymer material, and forming an amino-aldehyde resin coat as a wall on a surface of the hydrophobic core material in this system, wherein a polymer material containing an acrylamide-alkylsulfonic acid or a salt thereof as an essential monomer is used as the anionic water-soluble polymer material. Japanese Patent Publication No. 53539/1993 (JP-5-53539B) discloses a process for producing a microcapsule, which comprises emulsifying a hydrophobic and hardly volatile organic compound in an aqueous solution of an acrylic acid-methacrylic acid copolymer, and adding urea and/or melamine and formaldehyde to the resulting matter for polymerization, to form a coat of a copolymer such as a urea-formaldehyde copolymer or a melamine-formaldehyde copolymer around the organic compound. In such an in-situ polymerization method, however, since a large number of capsule particles without the coloring agent are produced as by-products in the encapsulation process, a step for removing not only an emulsifying and dispersing agent but also by-product particles is essential. Further, as the same as in the coacervation method, the particle size distribution depends on the dispersiveness of the core material.
Moreover, as the interfacial polymerization method, there is known a method for forming a wall of a capsule by polymerizing a polyhydric alcohol existing in a continuous aqueous phase and an isocyanate monomer existing in an oil phase of a core material at the interface surface. For example, Japanese Patent Application Laid-Open No. 000362/1994 (JP-6-000362A) discloses a process for producing a microcapsule, which comprises covering a hydrophobic liquid with a polyurea or a polyurethane resin by an interfacial polymerization, wherein the hydrophobic liquid in which a polybasic acid halide and a polyvalent isocyanate are dissolved is added and dispersed in an aqueous solution of a water-soluble polymer material having a hydroxyl group, an amino group or an imino group, and an alkaline compound is added thereto, and then a polyvalent amine or a polyhydric alcohol is added to the resulting mixture. Japanese Patent Application Laid-Open No. 343852/1994 (JP-6-343852A) discloses a process for producing a microcapsule whose wall comprises a polyurea or a polyurethane polyurea, wherein an oil-in-water droplet type emulsion is obtained from an aqueous solution and an oil-based solution by using a stirrer equipped with a saw-toothed impeller, and then the wall is formed. Japanese patent Publication No. 37975/1986 (JP-61-37975B) discloses a process for producing a microcapsule, which comprises emulsifying a hydrophobic liquid in which a polyvalent isocyanate is dissolved, into fine drops in an aqueous solution containing an emulsifying agent, and forming a coat on the interface, wherein the increase of the viscosity due to the preservation of the emulsified liquid is inhibited by adding a salt of a thio acid in the aqueous solution containing the emulsifying agent. Japanese Patent No. 2797960 discloses a process for producing a microcapsule, which comprises emulsifying a hydrophobic liquid in which a polyvalent isocyanate is dissolved, in an aqueous solution containing an emulsifying agent, and then forming a coat on the interface between the hydrophobic liquid and water, wherein the use of an acrylic multiple copolymer as a main component of the emulsifying agent can eliminate generation of formaldehyde and improves solvent resistance, heat resistance, pressure resistance, and others, of the microcapsule. Japanese Patent No. 3035726 discloses an oil-containing microcapsule in which a liquid lubricant is coated with a three-dimensional urethane polymer membrane. Such an interfacial polymerization has advantages that the formation of particles without a core material can be inhibited. However, in the interfacial polymerization, unreacted monomers remain in the oil phase and the aqueous phase, and electrophoretic properties of the colored fine particles is deteriorated due to highly polar isocyanate monomers remaining in the oil phase in use as an encapsulated ink for an electrophoretic display material. Further, as the same as in the encapsulation method by coacervation or in-situ polymerization, the particle size distribution depends on the dispersiveness of the core material.
Therefore, new microencapsulation techniques have been required for an electrophoretic particle microcapsulating ink, by which production of microcapsule particles without a coloring agent can be inhibited and the particle size can be controlled.
Incidentally, Japanese Patent Application Laid-Open No. 66600/1993 (JP-5-66600A) discloses, as a powdery toner for visualizing an electrostatic latent image, an encapsulated toner encapsulating a coloring agent within an anionic self-water dispersible resin. This document describes a copolymer, as an anionic self-dispersible resin, having 20 to 500 mg equivalent of an acid group (such as carboxyl group) per 100 g of a solid resin. Moreover, the document also discloses a process for production of the toner, which comprises dispersing a mixed composition containing the anionic self-water dispersible resin and a coloring agent, forming a capsulated particle in an aqueous medium by phase inversion emulsification (emulsification with phase inversion) of the mixed composition, and separating the produced capsule particle from the aqueous medium for dryness; and a process for producing a toner which comprises, after the phase inversion emulsification, hydrolyzing the neutralized acid group to form a free acid group. An organic solvent and water which are utilized for the phase inversion are removed from the formed encapsulated toner by drying, and the resultant toner is used for fixation on an object by heat-melting. Therefore, in the encapsulated toner, the coloring agent cannot be moved in the capsule. Moreover, crosslinking of the resin deteriorates the fixing property of the toner.